[feature] Add way to specify --seconds-per-day for arbitrary date ranges

[0jrp0]

This is kind of a crazy request that I'd like to work on and see what I can come up with. It's related to #238 but a more advanced way of specification.

My requirement is that I have a period from 1998 to 2007 that's pretty boring. When I pair it with music, it would be best if this period were compressed from the current runtime of 3:20s to 2:07s. In order for me to do that I would need to tell Gource to use a faster --seconds-per-day for that date range and another value for the date range after it.

Specifying this on the commandline is going to be difficult. One option is to have two parameters that are coupled with each other and that have comma separated specs. For example:

$ gource --seconds-per-day VAL1,VAL2 --seconds-per-day-dates DATE1,DATE2

Then in code, split these into two vectors. Make sure the size of them are the same, if not, throw error. If there's only one value in --seconds-per-day, then just implement current behavior.

Alternative 1: if it's too dangerous to reuse --seconds-per-day, define a new parameter.

Alternative 2: if command line specification is too hairy, another option is to specify this through a file just like --caption-file

Any preference on design?

[acaudwell]

Interesting idea.

I think can probably re use the --seconds-per-day parameter.

For the transition point maybe use a date and time rather than just a date. There's a parseDateTime() function for that.

Perhaps you only need to specify one date in the above case (the date it changes to the the next value), so --seconds-per-day-transitions has to have one less value than --seconds-per-day.

Possibly you may find you want to interpolate the --seconds-per-day values when you get to the transition so it doesn't immediately change values but ramps up.